Generally, a wireless communication system is developing to diversely cover a wide range to provide such a communication service as an audio communication service, a data communication service and the like. The wireless communication is a sort of a multiple access system capable of supporting communications with multiple users by sharing available system resources (e.g., bandwidth, transmit power, etc.). For example, the multiple access system may include one of CDMA (code division multiple access) system, FDMA (frequency division multiple access) system, TDMA (time division multiple access) system, OFDMA (orthogonal frequency division multiple access) system, SC-FDMA (single carrier frequency division multiple access) system and the like.
According to a related art, one transmission antenna and one reception antenna (single input single output (SISO)) are used. On the contrary, MIMO (multiple input multiple output) corresponds to a method of using a plurality of transmission antennas and a plurality of reception antennas. Transmission and reception efficiency of data can be enhanced by the method. In particular, if a transmitting end or a receiving end of a wireless communication system uses a plurality of antennas, capacity can be increased and performance can be improved. In the following description, MIMO can also be called multiple antennas.
In the multiple antenna technology, it may not depend on a single antenna path to receive a whole message. Data is completed by combining data fragments received from many antennas in one place in the multiple antenna technology instead. When the multiple antenna technology is used, a data transmission speed may be enhanced in a cell area having a specific size or a system coverage may be enlarged while a specific data transmission speed is secured. And, this technology is widely used in a mobile communication terminal, a relay station, and the like. According to the multiple antenna technology, a throughput limitation of a single antenna used by a conventional technology in a mobile communication can be overcome.
In a configuration of a general MIMO communication system, NT number of transmitting antenna is installed in a transmitting end and NR number of receiving antenna is installed in a receiving end. In case that both the transmitting end and the receiving end use plural number of antennas, a theoretical channel transmission capacity is increased compared to a case that the plural number of antennas are only used for either the transmitting end or the receiving end. The increase of the channel transmission capacity is proportional to the number of antenna. Thus, a transfer rate is enhanced and frequency efficiency is enhanced. If a maximum transfer rate is represented as Ro in case of using a single antenna, the transfer rate using multiple antennas can be theoretically increased as much as the maximum transfer rate Ro multiplied by a rate of increase Ri.
For instance, MIMO communication system using 4 transmitting antennas and 4 receiving antennas may be able to theoretically obtain the transfer rate of 4 times of a single antenna system. After the theoretical capacity increase of the multi-antenna system is proved in the mid-90s, various technologies for practically enhancing a data transmission rate have been actively studied up to date and several technologies among them are already reflected in such a various wireless communication standard as a next generation wireless LAN and the like.
A multi antenna system uses a plurality of transmitting antennas and a plurality of receiving antennas and may be able to overcome a fading impact occurring on a radio channel via a plurality of transmitting and receiving paths. Hence, data transmission speed and data transmission quality can be enhanced compared to a single antenna. Yet, in order to obtain high transmission speed in MIMO system, it is necessary to have a sufficient distance between antennas. Since a base station receives a signal using a wide coverage, it is able to install antennas including a sufficient distance between base stations. Yet, it is practically difficult for a miniaturized terminal to have a sufficient distance. Hence, multi-user MIMO (MU-MIMO) enabling communication between many terminals including a single antenna and a base station of multiple antennas is widely studying and the MU-MIMO is included in 3GPP standard such as LTE-advanced and the like.
In multi-cell environment, transmission speed and quality of a user positioned at a boundary of a cell is seriously deteriorated by inter-cell interference (ICI) caused by a neighboring cell. In order to overcome the ICI, it may be able to use a frequency reuse method corresponding to a method of reducing interference by allocating an orthogonal frequency resource between adjacent cells. Yet, the frequency reuse method may bring not only improvement of transmission speed and quality of a user positioned at a cell boundary but also degradation of transmission speed and quality of a whole network. In order to solve the aforementioned problem, as a method of reducing the ICI by efficiently utilizing a frequency resource, there exists a CoMP (coordinated multi-point) scheme corresponding to a scheme of reducing the ICI via coordination between multiple cells. The CoMP scheme forms a virtual MIMO system by exchanging channel information or data information of a user with each other between multiple cells.
In order to efficiently transceive data in MU-MIMO system, it is necessary to have channel information between antennas. In a time division cellular system, a base station allocates a pilot resource to a user equipment to obtain the channel information. The user equipment transmits a predetermined pilot sequence to the base station via the allocated pilot resource and the base station estimates channel information by receiving the pilot sequence from the user equipment. Although a demand of the pilot resource increases in proportion to the number of user equipment, an amount of the pilot resource is limitative. Hence, it is practically impossible to allocate the pilot resource orthogonal between multiple cells to all user equipments. Hence, it is difficult to perfectly estimate a channel of a user by using a pilot resource and a transmission and reception filter constructed by an estimated channel may cause unintended interference. Performance degradation caused by reusing a pilot resource is especially serious in a cooperative cellular environment.
Hence, in order to obtain high transmission speed and quality by suppressing interference in a multi cell cooperative communication system, it is mandatory to control interference caused by a pilot resource. In other word, a base station should allocate an optimized pilot resource to each user equipment, estimate a channel between user equipments using a given pilot resource and design a transmission and reception filter based on the estimated channel. In order to maximize transmission speed, it is important to design an optimized scheme between the aforementioned steps. Hence, it is required to have a method of designing an enhanced transmission and reception filter capable of cancelling inter-cell interference and/or inter-user equipment interference from an estimated channel.